Browsers continually evolve, and their features are always changing... sometimes gradually and sometimes massively. There were a number of modest feature changes that occurred between 11.64 and 12.14 - modest, that is, if one wasn't negatively affected by them. @Pesala is quite familiar with these, and he had at least one excellent thread on the subject of 11.64 that apparently did not survive the recent transfer to here of selected topics from the old My Opera forums.

In my case, I keep 11.52 on my system (as well as 12.14) because there are a few things that were changed in the transition from the 11.5x series to the 11.6x series that I didn't care for, back in the day. And (as @pesala has noted), there are a few websites that don't work well with the 12.xx series, so in my case I keep that 11.x version for such compatibility purposes. I prefer 12.14 over 12.15/16 primarily because it was the last Old Opera version that allowed a user to set and preserve his own choice of default search engine, a feature that matters greatly to me (but not to many others).

One thing to keep in mind is that the older the browser version being used, the greater the security-vulnerability population it may possess. If that is a concern to a user, that user needs to check more deeply into the specific risks by tracking down the version changelogs and the security alerts that describe each vulnerability in greater detail. However, Opera browsers have always been extremely good from a security standpoint, so the security differences between 11.64 and 12.1x are not that major - though, of course, even one 'hole' might contribute to something nasty somehow getting into a system. Using other security tools outside of the browser and practicing safe-hex will generally reduce any overall security threats to a very low level, even when using a browser of 11.64's vintage.

I don't have any problem setting a default search engine in 12.16, Maybe I don't understand what blackbird71 means?

Have you tried to install and set a custom search engine (such as StartPage) as the default, and have that setting persist after you turn Opera off and back on? Selecting a default from Opera's as-shipped list may allow the setting to persist, but if you add your own engine, you'll find it won't "stick" as default. As a preventive against malware hijacking the default search engine, Opera in 12.15 changed the behavior of the default engine setting to only allow a choice from Opera's "wired-in" engine listing, with the side effect that it also prevents a user inserting a custom engine and making it default. That same behavior was carried on into 12.16 as well. Because secure search engines like StartPage are used my many folks but aren't on Opera's as-shipped list of engines to choose from as default, a lot of people were (and are still) not happy with what Opera did.

I chose not to go into all the gory details in my earlier post (other than to indicate "his own choice"), since that wasn't the primary reason for posting it.

Oh yes I see what you mean now. I seem to remember thinking it must have been be a bug. Now though, with Opera 20 it's still the case that you can't change the default engine, and it's one of the most irritating things about it.

Now, in hindsight testing 12.16 again, I realised that I had set google.co.uk as the permanent default search and so thought that there was no problem, but when trying to change the default to StartPage as you suggested (which indeed didn't stick upon a restart), I then found that my google search domain had been changed back to google.com. However, I just tried DuckDuckGo and it seems to stick as the default. Perhaps it is sanctioned by Opera?

... I just tried DuckDuckGo and it seems to stick as the default. Perhaps it is sanctioned by Opera?

Perhaps. This all unfolded with 12.15, and I forget some of the discussion at the time, but I remember (misremember?) some users were indeed able to get DuckDuckGo to work. I don't recall why that was true, but because of the issue (and because I use StartPage as my default search engine), I elected to not apply the 12.15 update... and because 12.16 was merely a reissued 12.15, I avoided it as well. Now I believe the relevant threads from My Opera have evaporated, so I've got no way to go back and check those threads. If you've got a "stock" 12.15/16 version with regard to listed search engines, you might look in the list and see if DuckDuckGo came with it. If so, that's the explanation, since at the time, I recall that users were supposed to be able to default any engine in the as-shipped list.

There were also some other issues with 12.15 at the time (and sporadically reported since, in both 12.15/16) that involved some users having some stability issues with various sites as well, though again, I didn't pay too much mind to them since I had no plans to migrate from 12.14.

I'm unclear why people are having problems setting the default search engine in 12.16. I've got Bing set as my default in both the x86 and x64 builds of 12.16 (predominately use the x64 build, but there area few sites which do not work properly in it for some unknown reason). Maybe it's because I installed them as self-contained installs (portable, as opposed to standard install).

Both: 11.64 as default, and 12.14 for sites that don't work in 11.64 (I found two more sites that I need to use are broken in 11.64 now).

That was interesting, I didn't think you could have two versions side by side.

Not only two, you can have many versions side-by-side, running at the same time. I have 11.64, 12.16 (my new default), 20, Next, and Developer — all installed as USB installs. I can open any of the others from my right-click menu in 11 or 12.

Opera 12.16: Pings on using Alt Key to type — for example, but works on those two sites.

Opera 20: Works on all sites, PDF plugins OK, no mail client, no customisation, no custom buttons, menus, shortcuts, gestures, or skins, always a low-pitched humming noise while it's running. No tab-stacking, no sessions, no notes for boiler-plate text: etc.

[Abbr=1. Make sure that you are using the latest version of Opera. • 2. Search before posting. • 3. Use descriptive thread titles. • 4. Be specific and to the point. • 5. Include all relevant information. • 6. Don't hijack other people's threads. • 7. Report back if the problem is solved.]Guidelines[/abbr]

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So 11.x has less compatibility issues than 12.x? I wouldn't have thought that.

Sometimes different compatibility issues than 12.x... not necessarily less issues. I'm not sure anyone has really compared them by incompatibility count, but the reports I've run across indicate one may work where the other doesn't. But then, most Opera users have gotten used to that sort of thing over the years...

Not only two, you can have many versions side-by-side, running at the same time. I have 11.64, 12.16 (my new default), 20, Next, and Developer — all installed as USB installs. I can open any of the others from my right-click menu in 11 or 12.

Several versions installed on USB yes, but you can only have one version installed on your system drive?
I have a limited number of USB ports so I only connect a USB stick when I have to.

I don't like how tabs behave on version 12. Sometimes when I move the mouse pointer over a tab, it stretches out and become wider. It's like the tabs sometimes are extra narrow until the mouse gets near them, and it's not because I have many tabs, it can happen with only two tabs open. I don't remember which version introduced this, but it was not like this in version 9. I also thinks tabs looked nicer in v9.

Why this is annoying me? Because, sometimes when I try to close a background tab, I miss the X button because the tab suddenly becomes wider. If anyone could tell me the version this behavior was introduced, I would be thankful.

I don't know if it matters, but I'm using the "Opera Standard" skin. (I find it more classic than the "Opera Classic Skin".)

You kept 11.52 for a long time, you don't happen to remember in what way it was better than 11.64?

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Opera 12.16: Pings on using Alt Key to type — for example, but works on those two sites.

I never get a ping. If I press ALT and release it without pressing another key, the Opera menu opens. But no ping sound.

Several versions installed on USB yes, but you can only have one version installed on your system drive?
I have a limited number of USB ports so I only connect a USB stick when I have to.
USB Installation is a sysnonym for a stand-alone-installation. If you choose this installation, you can select any folder for use (and this can be, of course, a usb stick)